The present invention relates to a method for producing a nitrogen oxide storage material that contains at least one storage component in the form of particles of an oxide, carbonate or hydroxide of an element selected from the group consisting of magnesium, strontium, barium, lanthanum and cerium on a support material selected from the group consisting of doped cerium oxide, cerium/zirconium mixed oxide and aluminum oxide or mixtures of these.
In the area of gasoline engines the so-called lean-burning engines, which are operated with lean air/fuel mixtures when operating under a partial load, were developed to reduce fuel consumption. A lean air/fuel mixture contains a higher oxygen content than is necessary for complete combustion of the fuel. The exhaust gas then contains the oxidizing components oxygen (O2), and nitrogen oxides (NOx) in an excess amount compared to the reducing components carbon monoxide (CO), hydrogen (H2) and hydrocarbons (HC). A lean exhaust gas usually contains 3-15 vol % oxygen. However, at load or full load operation, even in the case of lean-running gasoline engines are operated with stoichiometric or even substoichiometric, i.e., rich, air/fuel mixtures, too.
Because of the high oxygen content of the exhaust gas of lean burning motors or diesel motors the nitrogen oxides contained in them may not be reduced to nitrogen by means of the so-called three-way catalysts with simultaneous oxidation of hydrocarbons and carbon monoxide, as is the case with stoichiometrically operated gasoline engines.
For this reason nitrogen oxide storage catalysts that store the nitrogen oxides in lean exhaust gas in the form of nitrates were developed to remove the nitrogen oxides from these exhaust gases.
The operation of nitrogen oxide storage catalysts is described in detail in the SAE publication SAE 950809 which is relied on and incorporated herein by reference. Accordingly, nitrogen oxide storage catalysts consist of a catalyst material that is applied for the most part in the form of a coating onto an inert ceramic or metal honeycomb, a so-called carrier element. The catalyst material contains the nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material again consists of the actual nitrogen oxide storage component that is deposited on a support material.
Chiefly, the basic oxides of the alkali metals, alkaline earth metals and rare earth metals, in particular barium oxide, which react with nitrogen dioxide to form the corresponding nitrates, are used as storage components. It is known that these materials in air occur for the most part in the form of carbonates and hydroxides. These compounds are also suitable for storage as nitrogen oxides. For this reason, when the term xe2x80x9cbasic storage oxidesxe2x80x9d is used within the scope of the invention, the corresponding carbonates and hydroxides are also included in this term.
The substances usually used as the catalytically active components are the noble metals of the platinum group, which as a rule are deposited onto the support material together with the storage component. Mainly active, high surface area aluminum oxide is used as support material.
The task of the catalytically active components is to convert carbon monoxide and hydrocarbons in the lean exhaust gas to carbon dioxide and water. In addition, they are intended to oxidize nitrogen monoxide contained in the exhaust gas to nitrogen dioxide, so that it can react with the basic storage material to form nitrates. The storage capacity of the storage material drops off with increasing deposition of nitrogen oxides in it and for this reason it has to be regenerated from time to time. To do this, the engine is run with stoichiometric or rich air/fuel mixtures for a short time. Under the reducing conditions in the rich exhaust gas the nitrates that are formed are broken down to nitrogen oxides NO, and, using carbon monoxide, hydrogen and hydrocarbons as reducing agents, reduced to nitrogen, forming water and carbon dioxide. The storage catalyst operates as a three way catalyst during this phase of operation.
Various combinations of storage components and support materials are known from the patent literature. For instance, EP 0 562 516 A1 describes a catalyst of barium oxide, lanthanum oxide and platinum on a support material of aluminum oxide, zeolite, zirconium oxide, aluminum silicate or silicon dioxide, where at least one part of the barium oxide and the lanthanum oxide forms a mixed oxide. This mixed oxide is intended to suppress the formation of lanthanum aluminate, which would otherwise lead to aging of the catalyst. To make the catalyst, a honeycomb carrier element is first coated with an aluminum oxide dispersion and then dried and calcined. Then the coating is impregnated sequentially or simultaneously with a lanthanum salt solution and a barium salt solution, dried and calcined at 300xc2x0 C. for a period of 1 h. Then the coating is impregnated with a platinum salt solution, again dried and calcined.
EP 0 653 238 A1 proposes the use of titanium oxide as support material, which contains at least one element from the group consisting of the alkali metals, alkaline earth metals and the rare earth metals in the form of a solid solution. To produce this storage material, titanium dioxide is impregnated with a solution of the precursor compounds of the storage components and then calcined at temperatures over 600xc2x0 C. to form the solid solution.
EP 0 666 103 A1 describes a catalyst that contains a nitrogen oxide storage component and a noble metal on a porous support material. Aluminum oxide, zeolite, zirconium oxide, aluminum silicate and silicon dioxide are proposed as support materials. The nitrogen oxide storage component and noble metal are deposited in close association on these support particles. In addition, the catalyst can also contain cerium oxide as an oxygen storage component, with the cerium oxide being kept separate from the noble metal and thus from the nitrogen oxide storage component. To deposit the storage components and the noble metal onto the support material it is impregnated with solutions of precursors of these components and calcined.
WO 97/02886 describes a nitrogen oxide storage catalyst in which metal oxides, metal hydroxides, metal carbonates and metal mixed oxides are used as storage components. The metals can be lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium or barium. The storage components can either be used in powder form or deposited on aluminum oxide by impregnation with precursor compounds of the storage components. The impregnated aluminum oxide powder is calcined for 2 h at 550xc2x0 C.
DE 197 13 432 A1 describes a catalytic base material for an exhaust gas purification catalyst that is obtained by impregnating cerium oxide powder with a barium-containing solution and calcining the cerium oxide particles at about 400-1100xc2x0 C. to form and accumulate barium oxides on the surface of the cerium oxide particles. According to this publication, a mixture of barium oxides and cerium oxide particles is heated to a relatively high temperature in order to intentionally form coarse-grained barium oxides on the surface of the cerium oxide particles. Temperatures of 800-1100xc2x0 C. are effective for this. The cerium oxide particles are preferably calcined at 900xc2x0 C. for a period of 24 h. Particle sizes of the barium oxide particles between 7 and 14 xcexcm result. At a calcination temperature of 200xc2x0 C. the barium oxide particles still have an average particle size of 1.9 xcexcm.
The catalytic base material in accordance with DE 197 13 432 A1 serves for making a catalyst that is particularly effective when it is used on an engine with lean combustion. This catalyst is thus a so-called lean NOx catalyst, which is capable in the presence of sufficient reducing components in the exhaust gas (carbon monoxide and hydrocarbons) of converting nitrogen oxides even in a constantly lean exhaust gas. The catalytic base material increases the temperature stability of the catalyst. DE 197 13 432 A1 does not provide any information regarding a possible nitrogen oxide storage capacity of the catalytic base material.
Thus with the known methods for making nitrogen oxide storage materials the selected support material as a rule is conventionally impregnated with soluble precursors of the storage components, then dried and calcined. As thorough research by the inventor showed, chromatographic effects occur during drying and calcining and these lead to the storage components aggregating on the surface of the support materials to form larger particles with diameters between several hundreds of nanometers up to micrometers. DE 197 13 432 A1 is an extreme example, in which the barium oxide particles have diameters of several micrometers and thus have sizes of the same order of magnitude as the support material that is used.
Another effect of the known production processes is the agglomeration and sintering of the powder particles of the storage material as a consequence of the intimate contact between the powder particles and of the high temperatures in the calcination ovens. For this reason the calcined material cannot be directly deposited onto the intended carrier element, but rather it has to be thoroughly ground in a slow and energy-intensive process before the coating operation.
Research by the inventors show that the dynamic storage behavior of the storage material prepared in this way is adversely affected by the relatively large particles of the storage components. Interaction with the exhaust gas is hindered by the low surface area of the coarse particles and the nitrogen oxides have to travel lengthy diffusion paths in the particles. This is also true for the regeneration of the storage components, so that this operation becomes slower with increasing storage component particle sizes. When used in a motor vehicle, this has the result that the rate of storage of nitrogen oxides drops off relatively rapidly and the regeneration of the storage materials has to be begun long before the theoretical storage capacity, which is a function of the total weight of the storage components, has been reached.
Also, the poisoning of the storage components by the formation of sulfates is subject to the just described dependencies on the particle sizes of the storage components. This does lead to slower poisoning, but the required desulfatization is likewise made distinctly more difficult through these relationships.
For this reason an object of this invention is to provide a method for making a nitrogen oxide storage material that is characterized by a high useful storage efficiency, good dynamics in the storage and regeneration cycles and by easy desulfatization.
The above and other objects of the invention can be achieved by a method for making a nitrogen oxide storage material that contains at least one storage component in the form of particles of an oxide, carbonate or hydroxide of the elements magnesium, strontium, barium, lanthanum and cerium on a support material selected from the group consisting of doped cerium oxide, cerium/zirconium mixed oxide and aluminum oxide or mixtures of these. The method is characterized by the fact that the support material is suspended in an aqueous solution of precursors of the storage components, this suspension is then introduced into a hot gas stream, the temperature of which is calculated so that, during a residence time of the suspension in the hot gas stream of less than one minute, the solvent of the suspension is evaporated and the precursors of the storage components are thermally broken down and converted into the storage components, before the storage material formed in this way becomes separated from the stream of hot gases.
Thus, in contrast to the known methods the storage material is not produced by slow drying and subsequent calcination, where these processes in some cases take several hours, but rather the suspension of the support material in the solution of precursors of the storage components is sprayed into a hot gas stream. The drying and calcination of the storage material that forms takes place, in this case in a period of less than one minute.
The hot gas stream is preferably generated by a burner and then sent through a reaction tube. The combination of burner and reaction tube is also called the reactor in what follows.